For wet-gas flows under very high gas-cut conditions (e.g. gas volume fraction (GVF)>99%), it has proven difficult to measure, in-line, very small amounts of liquids and the water-in-liquid ratio (WLR), by using dual-energy nuclear measurement technology. For subsea gas wells, for example, it is important to detect, close to the wellhead, the first onset of water and also to quantify water flow rate, in order to provide an early warning of hydrate plugging and corrosion to long subsea flowlines and to control the injection rate of hydrate and/or corrosion inhibitors.
Differentiation between different liquids in a gas stream, such as condensate and water in a gas-condensate well, is also invaluable for the economics of a gas well and for the planning of liquid handling facilities at surface. This is because condensates can provide a large portion of the economic value from a gas well (e.g. a gas well with a flow rate of up to 100 MMscf/day and with condensate-gas-ratio (CGR) up to 200 bbl/MMscf, will produce up to 20,000 bbl/day of condensate).
Most commercial wet-gas flowmeters use a differential pressure device plus another sensing technology to measure gas and liquid flow rates, for example as described in P Mehdizadeh et al. “Wet gas metering: Trends in applications & technical developments”, Society of Petroleum Engineers (SPE) 77351 and Ø.L.Bø, et al. “New compact wet gas meter Based on a microwave water detection technique and differential flow Measurement”, North Sea Flow Measurement Workshop, 22-25 Oct., 2002.
There have been two directions in developing high GVF wet-gas flow meters:                Development of correction factors for gas flow rate metering for single-phase gas flow meters (such as orifice, Venturi, V-cone, Coriolis and ultrasonic) when a small amount of liquid is present; and        Use of hybrid in-line multiphase flowmeters having gas-liquid separation technology, to handle very high GVF multiphase flows. This however tends to yield bulky and expensive devices.        
Extension of existing flow meters to 3-phase capability and quantification of the WLR of the liquid phase in a wet-gas stream are thus desirable.
Three techniques are currently used to determine holdup and/or WLR: single- or dual-energy nuclear, electromagnetic (microwave, capacitance-conductance) and optical (e.g. infrared).
Roxar has deployed microwave sensors for water holdup detection for subsea gas wells: see Ø.L.Bø, et al. referenced above.
Based on sensing narrow-band near-infrared (NIR) optical bulk transmission through an oil/water mixture flowing in a narrowed gap, eProduction Solutions have used the Red-Eye™ water-cut meter (of former Premium Instruments) for measuring the WLR of liquid-rich (oil/water) flows. The Red-Eye device is stated to be an embodiment of U.S. Pat. No. 6,076,049B and is essentially an oil-fraction meter; the light emission centred at 950 nm wavelength is substantially transmitted through water phase and gas phase and is substantially absorbed by oil phase. Light emission at a second wavelength of 1140 nm is also disclosed; this is substantially absorbed by oil content and water content, and substantially transmitted by gas content. The narrow band water-cut meter is said to yield a full range water-cut detection independent of entrained gas. To measure accurately the transmittance at 950 nm, detectors are used to measure the directly transmitted light across the narrow flow gap, and also the light scattered forward and backward across the gap.
However, the Red-Eye meter is stated to be unsuitable for high gas flow (see a description of the Red-Eye meter at http://www.ep-solutions.com/PDFs/eP_L/L_Red_Eye_Water-Cut_Meter.pdf).
The performance of the Red-Eye water-cut meter has been reported in SPE 84506 “High-Water Cut: Experience and Assessment in PDO” (SPE Annual Technical Conference and Exhibition held in Denver, Colo., U.S.A, 5-8 Oct. 2003). The water-cut accuracy of the Red Eye meter was found to be within +/−1% absolute in oil/water flow for water cut between 85% and 100% with a confidence level better than 90%. In oil/water/gas flow (GVF=<25%), the accuracy was within +/−2% absolute with a confidence level better than 90%. The Red Eye meter exhibited large error (up to 20% absolute) at lower water cut, and also at low flow rate (gross of 200 m3/d). The latter was due to the flow not being properly mixed.
U.S. Pat. No. 6,292,756 B1 discloses a narrow-band infrared water fraction meter for use with gas wells (and for liquid hydrocarbon flows). The narrow band light emission centered at a wavelength of approximately 1450 nm is substantially transmitted through the gas phase and condensate (liquid hydrocarbon) phase of a flow stream and is substantially absorbed by the water phase of the flow stream (this is consistent with the fluid optical spectral properties shown in FIG. 1). The narrow-band infrared water fraction meter is said to provide water fraction detection independent of entrained condensate. There are no published test results or commercial products for water fraction detection of wet-gas flows based on narrow-band optical transmission measurements at a wavelength around 1450 nm.
One particular problem associated with measurement of optical transmission through a multi-phase flow stream is the scattering of the optical beam by a number of different mechanisms, in addition to any absorption by the phases.